trait App extends DelayedInit {
//...
private val initCode = new ListBuffer[() => Unit]
/** The init hook. This saves all initialization code for execution within `main`.
* This method is normally never called directly from user code.
* Instead it is called as compiler-generated code for those classes and objects
* (but not traits) that inherit from the `DelayedInit` trait and that do not
* themselves define a `delayedInit` method.
* #param body the initialization code to be stored for later execution
*/
override def delayedInit(body: => Unit) {
initCode += (() => body)
}
//...
}
object CircuitMain extends App {
// You can write tests either here, or better in the test class CircuitSuite.
Circuit.andGateExample //line in question ?
}
Related Post why-if-i-extend-the-app-trait-in-scala-i-override-the-main-method
I can see that in the comment it is saying it will be called by compiler generated code.
and from the related post, it is saying that the scala compiler will do special things about DelayedInit, what made this DelayedInit trait so special ? is it language related feature? or we can do similar things to other traits as well ? if yes, how to do that.
Was pondering this myself the other day...
Yes, it's a language specific feature - check Section 5.1 "Delayed Initializaton" in the Scala Language Specification:
http://www.scala-lang.org/docu/files/ScalaReference.pdf
You can also confirm this checking out the compiler source code:
https://github.com/scala/scala/blob/946b76ad8b31b1fd74e2f8e1972c4a9159ac690a/src/reflect/scala/reflect/internal/StdNames.scala
// Compiler utilized names
...
val delayedInit: NameType = "delayedInit"
val delayedInitArg: NameType = "delayedInit$body"
And also:
https://github.com/scala/scala/blob/96df73d994097e3318d003ddef00194b711289a3/src/reflect/scala/reflect/internal/Definitions.scala
// classes with special meanings
lazy val StringAddClass = requiredClass[scala.runtime.StringAdd]
lazy val ScalaNumberClass = requiredClass[scala.math.ScalaNumber]
lazy val TraitSetterAnnotationClass = requiredClass[scala.runtime.TraitSetter]
lazy val DelayedInitClass = requiredClass[scala.DelayedInit]
def delayedInitMethod = getMemberMethod(DelayedInitClass, nme.delayedInit)
Related
case class Fruit(
apple: String,
banana: String
)
trait AppleComponent { self: HasDatabaseConfig[JdbcProfile] =>
import profile.api._
class Fruits(tag: Tag) extends Table[Fruit](tag, "fruits") {
def apple = column[String]("apple")
def banana = column[String]("banana")
// what is this line of code used for?
override def * = (apple,banana) <> (Fruit.tupled, Fruit.unapply)
def applePK = primaryKey("apple", apple)
}
protected lazy val Apples = TableQuery[Fruits]
}
I am new to Scala Slick ,so I want to know what does override def * = (apple,banana) <> (Fruit.tupled, Fruit.unapply) mean? I really can't find any document about it.
Also, why we need a trait here?
Table class has * method which should implements SELECT * FROM ... semantics. Since Slick cannot guess how you want to extract columns, you have to write this manually (using all columns in the order you want).
<> is just so that you will have a case class returned by * rather than a tuple.
You don't need trait here. This:
// module name
trait AppleComponent {
// self-type for dependency injection
self: HasDatabaseConfig[JdbcProfile] =>
// dependencies injected by mixing-in this trait:
protected lazy val Apples = TableQuery[Fruits]
}
is called a cake pattern. In general it is an anti-pattern (but ZIO promotes recently a specific way of using it).
I'd say whatever documentation or tutorial you are using is outdated by a few years.
SLS 5.1 says "Delayed Initializaton. The initialization code of an object or class (but not a trait) that follows the superclass constructor invocation and the mixin-evaluation of the template’s base classes is passed to a special hook, which is inaccessible from user code. Normally, that hook simply executes the code that is passed to it. But templates inheriting the scala.DelayedInit trait can override the hook by re-implementing the delayedInit method, which is defined as follows:"
def delayedInit(body: => Unit)
The ScallopConf command-line parser extends DelayedInit and using it according to the docs generates the warning Selecting value apples from class Conf, which extends scala.DelayedInit, is likely to yield an uninitialized value.
How should the following simple example be rewritten so the warning is not generated?
import org.rogach.scallop._
class Conf(arguments: Seq[String]) extends ScallopConf(arguments) {
val help = opt[Boolean](name = "help", short = 'h', descr = "Help me please.")
}
object Gen {
def main(args: Array[String]) {
val conf: Conf = new Conf(args)
if (conf.help()) {
println(s"""Usage: Gen [--help]""")
sys.exit(-1)
}
println("Do some work here")
}
}
Making help a lazy val and calling conf.afterInit() before accessing conf.help should clear the compiler warnings.
I have the following macro defining a class and returning an instance of that class (with Scala 2.10.2 and the macro plugin):
def test[T] = macro testImpl[T]
def testImpl[T : c.WeakTypeTag](c: Context): c.Expr[Any] = {
import c.universe._
val className = newTypeName("Test")
c.Expr { q"""
class $className {
def method = 1
}
new $className
"""}
}
When I call the macro:
case class Cat(name: String)
val t = test[Cat].method
I get the following error:
method method in class Test cannot be accessed in Test
val t = test[Cat].method
^
My overall goal is to use vampire methods and to use quasi-quotes to describe the generated class. How can I solve this error?
In my post on vampire methods I mention this workaround for this bug. For some reason you currently aren't able to see an anonymous class's methods on the instance returned from the macro unless you create a wrapper class that extends the class with the methods and return an instance of that, instead.
You're seeing the same bug from a slightly different angle. You've named the class with the methods you want to see on the returned instance's structural type, but you still need a wrapper. The following will work:
c.Expr { q"""
class $className {
def method = 1
}
new $className {}
"""}
Note that all I've done is add a pair of brackets to the line creating the instance, so that I get an instance of an anonymous class extending $className instead of just a $className.
I have no idea what's behind this bug, and I'm not sure if Eugene knows more. I did recently confirm that it's still around in the latest build of 2.11.
I have been reading about doing Dependency Injection in scala via the cake pattern. I think I understand it but I must have missed something because I still can't see the point in it! Why is it preferable to declare dependencies via self types rather than just abstract fields?
Given the example in Programming Scala TwitterClientComponent declares dependencies like this using the cake pattern:
//other trait declarations elided for clarity
...
trait TwitterClientComponent {
self: TwitterClientUIComponent with
TwitterLocalCacheComponent with
TwitterServiceComponent =>
val client: TwitterClient
class TwitterClient(val user: TwitterUserProfile) extends Tweeter {
def tweet(msg: String) = {
val twt = new Tweet(user, msg, new Date)
if (service.sendTweet(twt)) {
localCache.saveTweet(twt)
ui.showTweet(twt)
}
}
}
}
How is this better than declaring dependencies as abstract fields as below?
trait TwitterClient(val user: TwitterUserProfile) extends Tweeter {
//abstract fields instead of cake pattern self types
val service: TwitterService
val localCache: TwitterLocalCache
val ui: TwitterClientUI
def tweet(msg: String) = {
val twt = new Tweet(user, msg, new Date)
if (service.sendTweet(twt)) {
localCache.saveTweet(twt)
ui.showTweet(twt)
}
}
}
Looking at instantiation time, which is when DI actually happens (as I understand it), I am struggling to see the advantages of cake, especially when you consider the extra keyboard typing you need to do for the cake declarations (enclosing trait)
//Please note, I have stripped out some implementation details from the
//referenced example to clarify the injection of implemented dependencies
//Cake dependencies injected:
trait TextClient
extends TwitterClientComponent
with TwitterClientUIComponent
with TwitterLocalCacheComponent
with TwitterServiceComponent {
// Dependency from TwitterClientComponent:
val client = new TwitterClient
// Dependency from TwitterClientUIComponent:
val ui = new TwitterClientUI
// Dependency from TwitterLocalCacheComponent:
val localCache = new TwitterLocalCache
// Dependency from TwitterServiceComponent
val service = new TwitterService
}
Now again with abstract fields, more or less the same!:
trait TextClient {
//first of all no need to mixin the components
// Dependency on TwitterClient:
val client = new TwitterClient
// Dependency on TwitterClientUI:
val ui = new TwitterClientUI
// Dependency on TwitterLocalCache:
val localCache = new TwitterLocalCache
// Dependency on TwitterService
val service = new TwitterService
}
I'm sure I must be missing something about cake's superiority! However, at the moment I can't see what it offers over declaring dependencies in any other way (constructor, abstract fields).
Traits with self-type annotation is far more composable than old-fasioned beans with field injection, which you probably had in mind in your second snippet.
Let's look how you will instansiate this trait:
val productionTwitter = new TwitterClientComponent with TwitterUI with FSTwitterCache with TwitterConnection
If you need to test this trait you probably write:
val testTwitter = new TwitterClientComponent with TwitterUI with FSTwitterCache with MockConnection
Hmm, a little DRY violation. Let's improve.
trait TwitterSetup extends TwitterClientComponent with TwitterUI with FSTwitterCache
val productionTwitter = new TwitterSetup with TwitterConnection
val testTwitter = new TwitterSetup with MockConnection
Furthermore if you have a dependency between services in your component (say UI depends on TwitterService) they will be resolved automatically by the compiler.
Think about what happens if TwitterService uses TwitterLocalCache. It would be a lot easier if TwitterService self-typed to TwitterLocalCache because TwitterService has no access to the val localCache you've declared. The Cake pattern (and self-typing) allows for us to inject in a much more universal and flexible manner (among other things, of course).
I was unsure how the actual wiring would work, so I've adapted the simple example in the blog entry you linked to using abstract properties like you suggested.
// =======================
// service interfaces
trait OnOffDevice {
def on: Unit
def off: Unit
}
trait SensorDevice {
def isCoffeePresent: Boolean
}
// =======================
// service implementations
class Heater extends OnOffDevice {
def on = println("heater.on")
def off = println("heater.off")
}
class PotSensor extends SensorDevice {
def isCoffeePresent = true
}
// =======================
// service declaring two dependencies that it wants injected
// via abstract fields
abstract class Warmer() {
val sensor: SensorDevice
val onOff: OnOffDevice
def trigger = {
if (sensor.isCoffeePresent) onOff.on
else onOff.off
}
}
trait PotSensorMixin {
val sensor = new PotSensor
}
trait HeaterMixin {
val onOff = new Heater
}
val warmer = new Warmer with PotSensorMixin with HeaterMixin
warmer.trigger
in this simple case it does work (so the technique you suggest is indeed usable).
However, the same blog shows at least other three methods to achieve the same result; I think the choice is mostly about readability and personal preference. In the case of the technique you suggest IMHO the Warmer class communicates poorly its intent to have dependencies injected. Also to wire up the dependencies, I had to create two more traits (PotSensorMixin and HeaterMixin), but maybe you had a better way in mind to do it.
In this example I think there is no big difference. Self-types can potentially bring more clarity in cases when a trait declares several abstract values, like
trait ThreadPool {
val minThreads: Int
val maxThreads: Int
}
Then instead of depending on several abstract values you just declare dependency on a ThreadPool.
Self-types (as used in Cake pattern) for me are just a way to declare several abstract members at once, giving those a convenient name.
What I'd like to achieve is having a proper implementation for
def dynamix[A, B](a: A): A with B
I may know what B is, but don't know what A is (but if B has a self type then I could add some constraints on A).
The scala compiler is happy with the above signature, but I could not yet figure out how the implementation would look like - if it is possible at all.
Some options that came to my mind:
Using reflection/dynamic proxy.
Simplest case: A is an interface on Java level + I can instantiate B and it has no self type. I guess it would not be too hard (unless I run into some nasty, unexpected problems):
create a new B (b), and also a proxy implementing both A and B and using an invocation handler delegating to either a or b.
If B can not be instantiated I could still create a subclass of it, and do as it was described above. If it also has a self type I would probably need some delegation here and there, but it may still work.
But what if A is a concrete type and I can't find a proper interface for it?
Would I run into more problems (e.g. something related to linearization, or special constructs helping Java interoperability)?
Using a kind of wrapping instead of a mixin and return B[A], a is accessible from b.
Unfortunately in this case the caller would need to know how the nesting is done, which could be quite inconvenient if the mixing in/wrapping is done several times (D[C[B[A]]]) as it would need to find the right level of nesting to access the needed functionality, so I don't consider it a solution.
Implementing a compiler plugin. I have zero experience with it but my gut feeling is that it would not be trivial. I think Kevin Wright's autoproxy plugin has a bit similar goal, but it would not be enough for my problem (yet?).
Do you have any other ideas that might work? Which way would you recommend? What kind of "challenges" to expect?
Or should I forget it, because it is not possible with the current Scala constraints?
Intention behind my problem:
Say I have a business workflow, but it's not too strict. Some steps have fixed order, but others do not, but at the end all of them has to be done (or some of them required for further processing).
A bit more concrete example: I have an A, I can add B and C to it. I don't care which is done first, but at the end I'll need an A with B with C.
Comment: I don't know too much about Groovy but SO popped up this question and I guess it's more or less the same as what I'd like, at least conceptional.
I believe this is impossible to do strictly at runtime, because traits are mixed in at compile-time into new Java classes. If you mix a trait with an existing class anonymously you can see, looking at the classfiles and using javap, that an anonymous, name-mangled class is created by scalac:
class Foo {
def bar = 5
}
trait Spam {
def eggs = 10
}
object Main {
def main(args: Array[String]) = {
println((new Foo with Spam).eggs)
}
}
scalac Mixin.scala; ls *.class returns
Foo.class Main$.class Spam$class.class
Main$$anon$1.class Main.class Spam.class
While javap Main\$\$anon\$1 returns
Compiled from "mixin.scala"
public final class Main$$anon$1 extends Foo implements Spam{
public int eggs();
public Main$$anon$1();
}
As you can see, scalac creates a new anonymous class that is loaded at runtime; presumably the method eggs in this anonymous class creates an instance of Spam$class and calls eggs on it, but I'm not completely sure.
However, we can do a pretty hacky trick here:
import scala.tools.nsc._;
import scala.reflect.Manifest
object DynamicClassLoader {
private var id = 0
def uniqueId = synchronized { id += 1; "Klass" + id.toString }
}
class DynamicClassLoader extends
java.lang.ClassLoader(getClass.getClassLoader) {
def buildClass[T, V](implicit t: Manifest[T], v: Manifest[V]) = {
// Create a unique ID
val id = DynamicClassLoader.uniqueId
// what's the Scala code we need to generate this class?
val classDef = "class %s extends %s with %s".
format(id, t.toString, v.toString)
println(classDef)
// fire up a new Scala interpreter/compiler
val settings = new Settings(null)
val interpreter = new Interpreter(settings)
// define this class
interpreter.compileAndSaveRun("<anon>", classDef)
// get the bytecode for this new class
val bytes = interpreter.classLoader.getBytesForClass(id)
// define the bytecode using this classloader; cast it to what we expect
defineClass(id, bytes, 0, bytes.length).asInstanceOf[Class[T with V]]
}
}
val loader = new DynamicClassLoader
val instance = loader.buildClass[Foo, Spam].newInstance
instance.bar
// Int = 5
instance.eggs
// Int = 10
Since you need to use the Scala compiler, AFAIK, this is probably close to the cleanest solution you could do to get this. It's quite slow, but memoization would probably help greatly.
This approach is pretty ridiculous, hacky, and goes against the grain of the language. I imagine all sorts of weirdo bugs could creep in; people who have used Java longer than me warn of the insanity that comes with messing around with classloaders.
I wanted to be able to construct Scala beans in my Spring application context, but I also wanted to be able to specify the mixins to be included in the constructed bean:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xmlns:scala="http://www.springframework.org/schema/scala"
xsi:schemaLocation=...>
<scala:bean class="org.cakesolutions.scala.services.UserService" >
<scala:with trait="org.cakesolutions.scala.services.Mixin1" />
<scala:with trait="org.cakesolutions.scala.services.Mixin2" />
<scala:property name="dependency" value="Injected" />
<scala:bean>
</beans>
The difficulty is that Class.forName function does not allow me to specify the mixins. In the end, I extended the above hacky solution to Scala 2.9.1. So, here it is in its full gory; including bits of Spring.
class ScalaBeanFactory(private val beanType: Class[_ <: AnyRef],
private val mixinTypes: Seq[Class[_ <: AnyRef]]) {
val loader = new DynamicClassLoader
val clazz = loader.buildClass(beanType, mixinTypes)
def getTypedObject[T] = getObject.asInstanceOf[T]
def getObject = {
clazz.newInstance()
}
def getObjectType = null
def isSingleton = true
object DynamicClassLoader {
private var id = 0
def uniqueId = synchronized { id += 1; "Klass" + id.toString }
}
class DynamicClassLoader extends java.lang.ClassLoader(getClass.getClassLoader) {
def buildClass(t: Class[_ <: AnyRef], vs: Seq[Class[_ <: AnyRef]]) = {
val id = DynamicClassLoader.uniqueId
val classDef = new StringBuilder
classDef.append("class ").append(id)
classDef.append(" extends ").append(t.getCanonicalName)
vs.foreach(c => classDef.append(" with %s".format(c.getCanonicalName)))
val settings = new Settings(null)
settings.usejavacp.value = true
val interpreter = new IMain(settings)
interpreter.compileString(classDef.toString())
val r = interpreter.classLoader.getResourceAsStream(id)
val o = new ByteArrayOutputStream
val b = new Array[Byte](16384)
Stream.continually(r.read(b)).takeWhile(_ > 0).foreach(o.write(b, 0, _))
val bytes = o.toByteArray
defineClass(id, bytes, 0, bytes.length)
}
}
The code cannot yet deal with constructors with parameters and does not copy annotations from the parent class’s constructor (should it do that?). However, it gives us a good starting point that is usable in the scala Spring namespace. Of course, don’t just take my word for it, verify it in a Specs2 specification:
class ScalaBeanFactorySpec extends Specification {
"getTypedObject mixes-in the specified traits" in {
val f1 = new ScalaBeanFactory(classOf[Cat],
Seq(classOf[Speaking], classOf[Eating]))
val c1 = f1.getTypedObject[Cat with Eating with Speaking]
c1.isInstanceOf[Cat with Eating with Speaking] must_==(true)
c1.speak // in trait Speaking
c1.eat // in trait Eating
c1.meow // in class Cat
}
}